In recent years, the market of interchangeable-lens type camera systems (also referred to simply as “camera systems”, hereinafter) have been spreading rapidly. Such an interchangeable-lens type camera system includes: a camera body having an image sensor such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal-Oxide Semiconductor); and an interchangeable lens apparatus having an imaging optical system for forming an optical image on a light receiving surface of the image sensor. The interchangeable lens apparatus is attachable to and detachable from the camera body. Such a camera system is used for shooting not only still pictures but also moving pictures, and therefore, an interchangeable lens apparatus suitable for shooting of moving pictures as well as still pictures is popular.
Previously known focusing methods for the imaging optical systems include: a method in which the entirety of component lens elements are moved out; and a so-called floating method in which a front unit and a rear unit are independently moved.
Patent Literatures 1 and 2 each disclose a configuration in which a front unit and a rear unit are moved toward an object in focusing. Patent Literature 3 discloses a configuration in which, in focusing, a first lens element is fixed, and second and subsequent lens elements are moved. Patent Literature 4 discloses a configuration in which, in focusing, the entire optical system is moved out toward an object.
When shooting a moving picture by using an imaging optical system suitable for an interchangeable lens apparatus, it is necessary to perform auto-focusing continuously at a high speed. In order to perform auto-focusing continuously at a high speed, a method of repeating a series of operations as follows is known: oscillating a focusing lens unit in an optical axis direction at a high speed (referred to as “wobbling”, hereinafter); detecting, from an output signal of an image sensor, signal components in a predetermined frequency band within a part of an image region; obtaining an optimal position of the focusing lens unit, at which the focusing lens unit is in its in-focus state; and moving the focusing lens unit to the optimum position. When this auto-focusing method is adopted, the outer diameters of lens elements constituting the focusing lens unit must be minimized to minimize the weight of the lens elements, in view of portability and power consumption.